An uncompressed video source can be captured in either the progressive or the interlaced scan format. The progressive scan format refers to the case where all lines of samples represent the same instances in time. In contrast, the interlaced scan format refers to the case where alternate lines of samples are captured at different instances in time. FIG. 1 is a diagram showing a field picture pair. As shown in the diagram, lines of samples captured at an earlier time t form a top field picture. As shown in the diagram, lines of samples captured at a later time t+1 form a bottom field picture. A top field picture and a corresponding bottom field picture can altogether be referred to as a field picture pair. An uncompressed field picture can be coded in frame coding or field coding. In frame coding, the top and the bottom field pictures are coded together as a single picture. In the field coding, the top field picture is coded as a separate picture from the bottom field picture. Note that, hereinafter a field picture will be simply referred to as a field.
In general, most video coding algorithms, such as the ISO/IEC 14496-2 Part 10 International Standard, employ two main coding types. Intra picture coding is performed within a picture without referring to other pictures; thus, a picture containing only sample pixels coded in intra picture coding is called an intra picture or simply an I-picture. On the other hand, inter picture coding utilizes a temporal correlation between video pictures by referring to samples from other pictures. A single-predictive inter picture (hereinafter referred to as a P-picture) predicts from one other picture, while a bi-predictive inter picture (hereinafter referred to as a B-picture) predicts from two other pictures.
Depending on the necessities of the application, the three coded picture types (intra, single-predictive, bi-predictive) can be used flexibly as determined by the video encoder. The order of the coding and decoding process (hereinafter referred to as coding order) is generally not the same as the capture order of source picture or the output order from a decoder for display (hereinafter referred to as display order). The coding order is arranged such that coding or decoding process of inter-pictures is performed after other previously-coded pictures that are referenced in the coding or the decoding process (hereinafter referred to as reference pictures).    [Non-patent reference 1] ISO/IEC14496-2 Part 10 International Standard